Proof. Because of the linear properties of (1) we can confine us to the monomials f⁢(x):=xnassignfxsuperscriptxnf(x):=x^{n}. Then f′⁢(x)=n⁢xn-1superscriptfnormal-′xnsuperscriptxn1f^{{\prime}}(x)=nx^{{n-1}}. By the binomial theorem we have